The present invention relates, in general, to amplifier circuits, and more particularly, to a novel high efficiency RF amplifier circuit.
In the past, the communications industry had utilized hybrid circuit manufacturing techniques to build radio frequency (RF) power amplifiers. One type of RF power amplifier generally operated within a specified range of an RF center frequency or fundamental frequency. Among other things, it was desirable for such an RF amplifier to have a high efficiency at the fundamental operating frequency. A technique commonly referred to as second-harmonic peaking was typically utilized to improve the efficiency of such an amplifier. Second-harmonic peaking frequently was implemented by connecting a quarter-wavelength transmission line in series between the amplifier's output stage and the amplifier's output terminal. The quarter-wavelength transmission line was a section of transmission line having a length that was approximately one-quarter the wavelength of the fundamental frequency. This quarter-wavelength transmission line produced a short circuit at all frequencies that were even harmonics of the fundamental frequency and an open circuit at all odd harmonics of the fundamental frequency. As a result, the amplifier's output voltage contained only the fundamental frequency and odd harmonics, while the output current contained only the fundamental frequency and even harmonics. Therefore, output power (current times voltage) was generated only at the fundamental frequency. The quarter-wavelength transmission line that was required to improve the efficiency of the amplifier had the disadvantage of consuming a large area thereby increasing the amplifier's size and its cost.
Accordingly, it is desirable to have an RF amplifier that has a high efficiency, that does not utilize a quarter-wavelength transmission line, that has a small size, and that has a low cost.